1. Field of the Invention
The present invention relates to a method of sputtering high moment iron nitride Fexe2x80x94N based magnetic head layers or thin films that are resistant to hard axis annealing and more particularly to ferromagnetic shield and/or pole piece layers or thin films wherein loss of magnetic anisotropy upon annealing in the presence of a field directed along the hard axis of these layers or films is minimized.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field writes information in the form of magnetic impressions in circular tracks on the rotating disk.
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from the rotating magnetic disk. The spin valve sensor is located between nonmagnetic nonconductive first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. The second shield layer may also serve as the first pole piece layer for the write head or may be a separate layer that is separated from the first pole piece layer by a nonmagnetic separation layer. In the latter case the read write head is referred to as a piggyback head. When the second shield and first pole piece are a common layer the magnetic head is referred to as a merged head. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
A typical sequence of steps in the fabrication of the above read write head is to form the first shield layer (SI) by sputter deposition on a slider substrate wafer, sputter deposit the read gap layer on the first shield layer, sputter deposit the sensor, which includes the free, pinned and pinning layers, on the first read gap layer, sputter deposit hard bias and lead layers connected to the sensor, sputter deposit the second read gap layer on the sensor and the hard bias and lead layers, plate a second shield/first pole piece layer (S2/P1) on the second read gap layer if the head is a merged head, sputter deposit the write gap layer on the second shield/first pole piece layer in the pole tip region, form a first insulation layer (II) on the second shield/first pole piece layer (S2/P1) in a yoke region by photopatterning a first layer of photoresist followed by hardbaking the photoresist at a temperature of 232xc2x0 C. for 400 minutes, plate the write coil layer on the first insulation layer, form the second insulation layer (I2) on the coil layer by photopatterning a photoresist layer in the yoke region and hardbaking it at 232xc2x0 C. for 400 minutes, frame plate a second pole piece layer (P2) on the write gap layer and the second insulation layer (I2), and connect it to the second shield/first pole piece layer (S2/P1) in a back gap region, anneal a second pole piece layer (P2) at 232xc2x0 C. for 400 minutes, plate copper straps and studs, deposit and lap alumina overcoat, plate gold electrical connection pads, and perform a GMR reset anneal at 220xc2x0 for 5 minutes. A subset of the steps which are critical in determining the magnetic properties of the shield and pole layers is shown in the following Chart A along with magnetic fields employed in each step.
During formation of the various ferromagnetic layers of the read write head, each ferromagnetic layer is formed with a magnetic easy axis which is oriented parallel to the ABS by sputter depositing or plating the ferromagnetic layer in the presence of a magnetic field that is oriented parallel to the ABS. Each of these layers also has a hard axis which is 90xc2x0 to the easy axis and has a magnetic anisotropy (HK) which is the amount of applied field required to rotate a magnetic moment of the ferromagnetic layer from the easy axis to the hard axis, which rotation magnetically saturates the ferromagnetic layer. It is desirable that the ferromagnetic layers have a high magnetic anisotropy for improving their performance in the magnetic read write head. After fabrication the easy axis of each of the first shield layer (S1), the second shield/first pole piece layer (S2/P1) and the second pole piece layer (P2) is oriented parallel to the ABS. It is important that subsequent processing steps not alter the easy axis orientation of the first shield layer (S1) and the second shield/first pole piece layer (S2/P1) so that a bias point of the sensor is not changed by nonparallel magnetic fields from these ferromagnetic layers. Further, it is important that the easy axis of each of the second shield/first pole piece layer (S2/P1) and the second pole piece layer (P2) be parallel to the ABS so that a write field current from the write coil rotates the magnetic moments of these free layers from the parallel position to effectively write a write signal into a track on the rotating magnetic disk.
As shown in the second column of Chart A, various field orientations are employed during various annealing steps shown in the first column of Chart A. After sputter depositing the first shield layer (S1) the wafer is annealed at a temperature of 475xc2x0 C. in the presence of a magnetic field parallel to the easy axis of the first shield layer which is also parallel to the ABS. This annealing sets the easy axis of the first shield layer parallel to the ABS. After fabricating the sensor, the wafer is annealed at a temperature of 220xc2x0 C. for 5 minutes in the presence of a field that is transverse to the easy axis of the first shield layer and to the ABS for the purpose of positioning a magnetic moment of the pinned layer perpendicular to the ABS which, in turn, orients magnetic spins of the pinning layer perpendicular to the ABS. When the annealing and transverse field are terminated the magnetic spins of the pinning layer pin the magnetic moment of the pinned layer perpendicular to the ABS. It should be noted that the field employed during this annealing step is along the hard axis of the first shield layer (S1) which is noted in parenthesis in the second column of Chart A. The second shield/first pole piece layer (S2/P1) is frame plated in the presence of a field that is oriented parallel to the ABS for setting its easy axis in that direction. During hardbaking of the first and second insulation layers (I1 and I2) a magnetic field of approximately 1000 Oe is employed transverse to the easy axes of S1 and S2/P1 for maintaining an exchange coupling field between the pinning and pinned layers of the sensor so that the magnetic moment of the pinned layer will be strongly pinned in the transverse direction. The fields during the hardbake of the insulation layers are directed along the hard axes of the first shield layer (S1) and the second shield/first pole piece layer (S2/P1). After plating the second pole piece layer (P2) the second pole piece layer is annealed at a temperature of 232xc2x0 C. for 400 minutes in the presence of a field which is transverse to the easy axes of S1, S2/P1 and P2 for the purpose of relaxing the stress in the second pole piece layer (P2). This field is directed along the hard axes of the first shield layer (S1), the second shield/first pole piece layer (S2/P1) and the second pole piece layer (P2). Finally, the magnetic spins of the pinning layer are reset at a temperature of 220xc2x0 C. for 5 minutes in the presence of a field that is directed transverse to the ABS and along the hard axes of the first shield layer (S1), the second shield/first pole piece (S2/P1) and the second pole piece layer (P2).
For GMR heads, transverse annealing is needed for optimal sensor performance, but this hard axis annealing may well have an adverse effect on the magnetic anisotropy and easy axis orientation of the shields and poles. In the plated 80/20 NiFe films used in the current process, for example, a single insulation hardbake on the hard axis nearly destroys the magnetic anisotropy, and the GMR reset process by itself causes significant degradation.
In the future, the use of high moment Fe-based films is highly desirable in the write head because (1) high moment is required to write at high density on high coercivity disks; and (2) favorable domain configurations and eddy current reduction for high frequency writing can be obtained in laminations with an insulating material. High Fe contents films are also attractive as shields because: (1) the thermal conductivity is comparable to the NiFe, which is advantageous for heat dissipation; and (2) high moment (4xcfx80MS=20 kG) makes the shield less susceptible than, for example, 80/20 NiFe (10 kG) or Sendust (9 kG) to the effects of stress induced anisotropy on easy axis orientation (i.e., the intrinsic anisotropy energy 1/2HKMS is higher relative to the stress induced anisotropy energy 3/2xcexS"sgr"). The favorable performance of Fe-based films as either poles or shields depends on controlling the easy axis orientation and domain structure in the films. We have observed, however, that Fe-N/alumina laminates, with structures such as (25 xc3x85 alumina/500 xc3x85 Fexe2x80x94N)6x/25 xc3x85 alumina=3175 xc3x85 or (18 xc3x85 alumina/1500 Fexe2x80x94N) 2x/25 xc3x85 alumina 3061 xc3x85 do no retain magnetic anisotropy when subjected to hard axis annealing. Similar to 80/20 NiFe, a single hardbake at 232xc2x0 C., 400 min causes the films to become nearly isotropic or even switch easy axis directions. Domain imaging by Kerr microscopy confirms that hard axis annealed Fexe2x80x94N/alumina laminates develop undesirable domain structures for either pole or shield applications.
Adding a metal M (M=Ti, Zr, Hf, Nb, Ta, B, Al, Si, etc.) to Fexe2x80x94N is known to increase thermal stability. Viala et al. (J. Appl. Phys. 81 (1997) 4498) report, however, that the easy axis orientation in Fe94.2Ta3.3N2.5 (at %) films deposited by DC magnetron sputtering rotates to the original hard axis direction after a 60 min hard axis anneal at 150xc2x0 C. These same films are stable under easy axis annealing up to 300xc2x0 C.
U.S. Pat. No. 5,473,492 (TDK, 1995 xe2x80x9cMagnetic Head Including a Reproducing Head Utilizing a Magnetoresistance Effect and Having a Magnetic Shielding Film Containing Nitrogenxe2x80x9d) describes use of Fexe2x80x94Mxxe2x80x94Ny (0.1 less than x less than 25, 0.1 less than y less than 25 (atomic %)) as S1 and/or S2/P1. The teaching in this patent does not address the issue of hard axis annealing, most likely because in 1995, the annealing requirements for GMR sensors were not understood. On the subject of processing conditions, the patent states (col. 4, 32-35): xe2x80x9cThe sputtering mode is not critical and the sputtering apparatus used is not limited and may be a conventional one. The operating pressure is usually about 0.1 Pa to about 10 Pa. Sputtering conditions including input voltage may be suitably determined in accordance with the sputtering mode.xe2x80x9d This patent clearly teaches that sputtering method and conditions are not critical for achieving soft properties in these films. While the sputtering conditions are less critical for some methods (e.g., RF magnetron and RF diode) than others (DC magnetron), this teaching is consistent with other literature and patents, which assume that the films will be annealed with a magnetic field along the easy axis.
The work of Viala et al. shows that film properties and stability after easy axis annealing cannot be used to predict the stability in a hard axis anneal. Viala concludes xe2x80x9cThe instability of interstitial N atoms, which are believed to control the magnetic anisotropy in these films, may be a cause for concern in device applications of this class of materialsxe2x80x9d. Loss of magnetic anisotropy and easy axis orientation in iron rich, high moment Fexe2x80x94Mxe2x80x94N films under hard axis annealing is a serious problem that compromises the usefulness of these materials in practical GMR recording heads.
Contrary to the teaching in the literature, I have found that at least some Fexe2x80x94Mxe2x80x94N films and laminations of these films can be fabricated which retain their magnetic anisotropy and easy axis orientation after the hard axis annealing conditions encountered in the GMR wafer process. Contrary to the teaching in U.S. Pat. No. 5,473,492, the sputtering conditions play a critical role in determining the ability of the film to withstand hard axis annealing.
I have provided a method of making at least one of the first shield layer (S1), the second shield/first pole piece layer (S2/P1) and the second pole piece layer (P2) which improves the properties of these layers so that upon hard axis annealing (annealing in the presence of a magnetic field directed along the hard axis of these layers) the loss of magnetic anisotropy is significantly less than such layers made by prior art processes. My method of making is preferably practiced with an RF magnetron sputtering system wherein the system includes a sputtering chamber, a wafer substrate in the chamber where the layers are to be formed, a magnetron cathode assembly in the chamber which includes a target of a selected material to be sputtered and a magnetron array behind the target with the target located between the magnetron array and the substrate, a first power supply for applying power with an RF component to the target, a second power supply for applying an RF substrate bias to the wafer substrate and a gas supply and means for supplying controlled mixtures of selected gases to the chamber under a specified pressure. The steps of making one of the ferromagnetic layers includes providing a selected material for the cathode target that is iron (Fe) based, employing the second power supply source to apply an RF substrate bias from 0 to xe2x88x9215 volts to the wafer substrate, employing the gas supply and chamber pressure control to supply selected gases to the chamber with a pressure from 4.0xc3x9710xe2x88x923 to 8.0xc3x9710xe2x88x923 mbar with at least one of the selected gases being nitrogen (N2) and sputter depositing the selected material to form on the wafer substrate the ferromagnetic layer with an iron nitride (Fexe2x80x94N) based material composition. Preferred alloy additions in Fexe2x80x94Mxe2x80x94N films are aluminum (Al) or zirconium (Zr). The substrate bias is preferably in a range from xe2x88x925 to xe2x88x9212 volts and a partial pressure of the nitrogen (N2) may be from 0.10xc3x9710xe2x88x923 to 0.40xc3x9710xe2x88x923 mbar. In still a further preferred embodiment magnetron sputtering is employed for sputter depositing multiple films of selected materials onto the substrate to form at least one of the ferromagnetic layers into a laminated layer of alternating iron nitride (Fexe2x80x94N) based and alumina films. In the preferred embodiment the above process conditions of the RF magnetron sputtering system are adjusted such that after hard axis annealing, the magnetic anisotropy HK of the ferromagnetic layer is at least 2.0 Oe and that a ratio of hard axis to easy axis coercivity is less than or equal to 0.60.
An object of the present invention is to provide a method of making a ferromagnetic layer of a read write head which has a minimal loss of magnetic anisotropy upon hard axis annealing.
Another object is to provide a method of making a ferromagnetic layer of a read write head which has a magnetic anisotropy of at least 2.0 Oe and a hard axis to easy axis coercivity ratio of less than or equal to 0.60 after hard axis annealing.
A further object is to employ the above processes for making a microstructurally different ferromagnetic layer in a read write head which has minimal loss of magnetic anisotropy after hard axis annealing.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.